Currently, martensitic stainless steel containing 12.0% to 14.0% by mass of Cr has been generally and widely used as steel for blades, which is used as razors and the like. The martensitic stainless steel is hardened to 620 HV to 650 HV, which is a range of hardness for razor blades, by heat treatments including quenching and tempering. The martensitic stainless steel is superior to high carbon steel in terms of rust prevention and corrosion resistance.
The martensitic stainless steel for razors is usually produced by a combination of hot rolling, cold rolling, and annealing, to be supplied to a subsequent process as razor steel in the form of a strip. In the subsequent process, after being subjected to die cutting, the martensitic stainless steel undergoes heat treatments including quenching and tempering by a continuous furnace and then undergoes blade edging and surface finishing to be produced as a final product.
The metal composition after the martensitic stainless steel is annealed is in a state in which carbides are dispersed in the ferrite structure. The granularity and the state of distribution of the carbides greatly influence the properties of the martensitic stainless steel as a razor blade having undergone heat treatment.
A large number of proposals have been made for stainless steel for razors. Among others, JP 3354163 B (Patent Document 1) by the applicant of the present invention may be mentioned as an invention that significantly improves quenchability by increasing the number of carbides. This Patent Document 1 discloses stainless steel for razors with excellent short-time quenchability consisting of, by mass, 0.55% to 0.73% C, not more than 1% Si, not more than 1% Mn, 12% to 14% Cr, and the balance of Fe with impurities and having a carbide concentration of 140 to 600 carbides/100 μm2 in a state of annealing by a continuous furnace. Note that the concentration of carbides shown in Patent Document 1 was obtained by measuring in a strip of stainless steel for razors prior to or during cold rolling in a state in which the steel is inserted into a continuous furnace set to a temperature of or higher than the Ac1 transformation temperature of the steel to be annealed.
JP 06-145907 A (Patent Document 2), which has been proposed by the applicant of the present application, discloses an invention of stainless steel for razors with an excellent quenchability consisting of, by mass, 0.55% to 0.73% C, not more than 1.0% Si, not more than 1.0% Mn, 12% to 14% Cr, 0.2% to 1.0% Mo, not more than 1.0% Ni, and the balance of Fe with impurities and having a carbide concentration of 140 to 200 carbides/100 μm2 in an annealed state thereof.